Near-field measurement systems are widely used for the characterization of large and/or low frequency antennas for which far-field or compact range measurement systems become too costly and in some cases impractical to use. The compact size of these near-field measurement systems allows for integration of measurement probes in planar, circular, and other array configurations. If the measurements are done by an array of sensors, the mechanical movements of the antenna-under-test (AUT) or device under test (DUT) are reduced or eliminated, and hence the measurement time is reduced. In the case of planar near-field measurements, the simplicity of the near-field to far-field transformation algorithms along with the array based approach results in almost real-time far-field characterization of the AUT in a hemisphere. A further reduction in measurement system size can be achieved if measurements can be done in the reactive region of the near-field, which is traditionally avoided because of the potential for reflection and coupling with the antenna.
Measurement in this reactive region is sometimes called very near-field measurement. An example of such a planar very-near-field antenna measurement system is the RFxpert® product (EMSCAN Corporation, Canada), which comprises an array of 1600 rapidly switchable probes printed on a 45×45 cm printed circuit board (PCB). The orthogonal H-field components (magnitude and phase) measured by the probes are transformed to a far-field pattern in a hemisphere using a plane-wave-spectrum (PWS) expansion. Despite the benefit of extremely fast electronic switching between the probes (rather than mechanical movements of the AUT in a chamber), the presence of the printed array of probes in the antenna's very near-field region creates a mutual coupling effect, which alters the fields around the antenna in at least two ways. First, a ground plane effect arises because the scanner PCB has an array of half-loop probes on a solid ground plane, which disturbs the near-fields by introducing a discontinuity in the medium and imposes new boundary conditions. Second, a mutual coupling effect arises because the scanner PCB and the individual measurement probes load the AUT, altering its radiation performance; for example, altering its input impedance. The mutual coupling effect is AUT-dependent and is difficult to compensate for. When measuring in the very-near-field region, it is not desirable to ignore the loading effect of the scanner PCB on the AUT, particularly at low frequencies, such as below 1 GHz, for example.
Therefore, there is a need in the art for systems and methods for mitigating or de-embedding these effects from the very near-field or reactive near-field measurements, in an effort to improve the accuracy of the resulting far-field results.